I have some Official Guides from the 20th century, and a few reprints from 1868, 1869, and 1875. Various issues were reprinted over time by the original publisher* and may be available somewhere on the net. I believe the New York Public Library has a complete set going back to the earliest days. The National Railway Historical Society has a fairly representative assortment, including some from prior to November 1883, when the railroads adopted the four Standard Time Zones; prior to that each railroad decided what city's sun time would be its standard, and their schedules make for interesting reading. However, the NRHS collection was transferred to the DeGolyer Library at Southern Methodist University in Dallas, where it is available for public use, but the DeGolyer is not in a position to do research on request, as the NRHS used to do.
* After the Guide largely lost its usefulness after Amtrak the publisher sold out to another company which still publishes a monthly freight-service guide, but I discovered on inquiry that they have absolutely zero interest in historical preservation and have nothing left of the original Guides.
Finding any Equipment Registers would be a catch-as-catch-can situation; the NRHS collection has a dozen or so over a period of several decades, but they are about 9x12" in size, an inch or more thick, and came out twelve times a year, constituting a serious space problem, and railfans in general have not been interested in collecting them. They're very useful if you're looking for a specific answer about something, but they don't provide the recreational reading and browsing afforded by the Official Guides and generally were not collected and saved as the Guides were.
If somebody on here knows of a source anywhere on the net capable of being searched, I'd certainly like to hear about it.